U.S. patent number 10,857,255 [Application Number 15/907,374] was granted by the patent office on 2020-12-08 for absorbent core wrap with a low basis weight nonwoven in the bottom layer.
This patent grant is currently assigned to The Procter & Gamble Company. The grantee listed for this patent is The Procter & Gamble Company. Invention is credited to Stefan Floter, Geeske Gratzel, Torsten Lindner.
United States Patent |
10,857,255 |
Floter , et al. |
December 8, 2020 |
Absorbent core wrap with a low basis weight nonwoven in the bottom
layer
Abstract
An absorbent article includes an absorbent core having an
absorbent material enclosed in a core wrap having a top layer and a
bottom layer. The bottom layer of the core wrap comprises a
nonwoven comprising synthetic fibers, which has a basis weight from
about 6 g/m.sup.2 to about 10 g/m.sup.2 and the nonwoven has a
static coefficient of friction as measured in cross-machine
direction of no more than 0.40.
Inventors: |
Floter; Stefan (Euskirchen,
DE), Gratzel; Geeske (Euskirchen, DE),
Lindner; Torsten (Euskirchen, DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
The Procter & Gamble Company |
Cincinnati |
OH |
US |
|
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Assignee: |
The Procter & Gamble
Company (Cincinnati, OH)
|
Family
ID: |
1000005228167 |
Appl.
No.: |
15/907,374 |
Filed: |
February 28, 2018 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20180256772 A1 |
Sep 13, 2018 |
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Foreign Application Priority Data
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Mar 9, 2017 [EP] |
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17160193 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
D04H
3/16 (20130101); D04H 5/00 (20130101); A61F
13/535 (20130101); A61L 15/50 (20130101); A61F
13/15634 (20130101); A61F 13/15674 (20130101); A61F
13/53704 (20130101); D04H 3/007 (20130101); A61L
15/20 (20130101); A61F 13/53756 (20130101); A61F
13/84 (20130101); A61F 13/15203 (20130101); A61F
2013/8455 (20130101); A61F 2013/15406 (20130101); A61F
2013/5315 (20130101); A61F 2013/530489 (20130101); A61F
2013/5349 (20130101); A61F 2013/530379 (20130101) |
Current International
Class: |
A61F
13/15 (20060101); D04H 5/00 (20120101); D04H
3/007 (20120101); D04H 3/16 (20060101); A61L
15/20 (20060101); A61L 15/50 (20060101); A61F
13/84 (20060101); A61F 13/535 (20060101); A61F
13/537 (20060101); A61F 13/531 (20060101); A61F
13/53 (20060101); A61F 13/534 (20060101) |
Field of
Search: |
;604/367,370,372,378,385.23,385.101 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1298240 |
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Apr 2003 |
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EP |
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WO 2015/075632 |
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May 2015 |
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WO |
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Other References
European Search Report, dated Jul. 8, 2017, Appln. No.
17160193.3-1308. cited by applicant .
International Search Report and Written Opinion, PCT/US2018/19807,
dated Apr. 25, 2018. cited by applicant.
|
Primary Examiner: Stephens; Jacqueline F
Attorney, Agent or Firm: Best; Christian M.
Claims
What is claimed is:
1. An absorbent article comprising: a liquid permeable topsheet; a
liquid impermeable backsheet; and an absorbent core at least
partially between the topsheet and the backsheet, wherein the
absorbent core comprises an absorbent material enclosed in a core
wrap comprising a top layer and a bottom layer, wherein the top
layer comprises a surfactant to increase hydrophilicity of the top
layer, wherein the bottom layer of the core wrap comprises a
nonwoven comprising synthetic fibers, wherein the nonwoven has a
basis weight from about 6 g/m.sup.2 to about 10 g/m.sup.2, wherein
the fibers of the nonwoven comprise a slip agent that reduces a
static coefficient of friction of the nonwoven, and wherein the
static coefficient of friction of the nonwoven matches a static
coefficient of friction of the top layer.
2. The absorbent article according to claim 1, wherein the slip
agent is a melt additive.
3. The absorbent article according to claim 2, wherein the melt
additive is a fatty acid amide.
4. The absorbent article according to claim 3, wherein the fatty
acid amide is selected from Oleamide, Erucamide, Stearamide and
mixtures thereof.
5. The absorbent article according to claim 3, wherein the nonwoven
comprises more than 0.05% of the fatty acid amide, by weight of the
nonwoven.
6. The absorbent article according to claim 5, wherein the nonwoven
comprises more than 0.1% of the fatty acid amide, by weight of the
nonwoven.
7. The absorbent article according to claim 3, wherein the nonwoven
comprises less than 1% of the fatty acid amide, by weight of the
nonwoven.
8. The absorbent article according claim 7, wherein the nonwoven
comprises up to 0.6% of the fatty acid amide, by weight of the
nonwoven.
9. The absorbent article according to claim 3, wherein the nonwoven
is a multilayer nonwoven comprising at least one spunbond layer and
at least one meltblown layer, and wherein the spunbond layer
comprises the fatty acid amide.
10. The absorbent article according to claim 9, wherein the
nonwoven is a SMS or SMMS nonwoven.
11. The absorbent article according to claim 1, wherein the
nonwoven comprises from about 0.2% to about 0.5% of erucamide, by
weight of the nonwoven.
12. The absorbent article according to claim 1, wherein the top
layer of the core wrap is a second nonwoven having a second basis
weight from about 6 g/m.sup.2 to about 10 g/m.sup.2.
13. A process for making an absorbent core, wherein the process
comprises: adding a melt additive to a plastic melt, wherein the
melt additive is a coefficient of friction reduction agent; making
a nonwoven comprising fibers from the plastic melt; converting the
nonwoven in a continuous process on a converting line into a
portion of the absorbent core, wherein the nonwoven is used to form
a bottom layer of a core wrap of the absorbent core, the absorbent
core comprising an absorbent material and a top layer, the top
layer and the bottom layer forming the core wrap around the
absorbent material; and treating the top layer with a surfactant to
increase a hydrophilicity of the top layer and such that a static
coefficient of friction of the nonwoven matches a static
coefficient of friction of the top layer.
14. The process according to claim 13, comprising: heat treating
the nonwoven so that diffusion of the melt additive at a surface of
the fibers of the nonwoven is accelerated and the coefficient of
friction is further reduced.
15. An absorbent article having a wearer-facing surface, the
absorbent article comprising: a liquid permeable topsheet; a liquid
impermeable backsheet; and an absorbent core at least partially
between the topsheet and the backsheet, wherein the absorbent core
comprises an absorbent material enclosed in a core wrap comprising
a first layer and a second layer, wherein the first layer is
positioned more proximate to the wearer-facing surface than the
second layer, wherein the second layer of the core wrap comprises a
nonwoven comprising synthetic fibers, wherein the nonwoven has a
basis weight from about 6 g/m.sup.2 to about 10 g/m.sup.2, wherein
the first layer and the nonwoven have a static coefficient of
friction as measured in cross-machine direction of no more than
0.40, as measured according to ASTM D1894-01, and wherein at least
some of the fibers of the nonwoven comprise a slip agent that
reduces the static coefficient of friction of the nonwoven.
16. The absorbent article according to claim 15, wherein the slip
agent is a melt additive.
17. The absorbent article according to claim 16, wherein the
nonwoven is a SMS or SMMS nonwoven.
18. The absorbent article according to claim 17, wherein the first
layer of the core wrap is a second nonwoven having a basis weight
from about 6 g/m.sup.2 to about 10 g/m.sup.2 and comprises a
surfactant to increase its hydrophilicity.
Description
CROSS REFERENCE TO RELATED APPLICATION
This application claims priority, under 35 U.S.C. .sctn. 119, to
Patent Application No. EP 17160193.3, filed on Mar. 9, 2017, which
is hereby incorporated by reference in its entirety.
FIELD
The present disclosure relates to absorbent articles for personal
hygiene, such as diapers, that comprise an absorbent core having an
absorbent material enclosed by a core wrap. The core wrap comprises
a top layer and a bottom layer. The bottom layer of the present
disclosure comprises a low basis weight nonwoven material having a
low coefficient of friction. A slip agent, in particular a fatty
acid amide melt additive, may be used to reduce the coefficient of
friction of the nonwoven.
BACKGROUND
Absorbent articles for personal hygiene such as diapers for babies
or incontinent adults are designed to absorb and contain body
exudates, in particular large quantity of urine. These absorbent
articles comprise several layers providing different functions,
such as a topsheet, a backsheet and in-between an absorbent core,
among other layers. The absorbent core should absorb and retain the
exudates for a prolonged amount of time in order to keep the wearer
dry and avoid soiling of clothes or bed sheets. At the same time,
the absorbent core should make the most efficient use possible of
the absorbent material to save material costs and keep the diapers
as thin as possible.
The majority of currently marketed absorbent articles comprise as
absorbent material a blend of cellulose fibers with superabsorbent
polymers (SAP) particles, also called absorbent gelling materials
(AGM), see for example U.S. Pat. No. 5,151,092 (Buell). Absorbent
cores with absorbent material consisting essentially of SAP without
cellulose fibers (so called "airfelt-free" cores) have also been
proposed. For example WO2008/155699 (Hundorf et al.) discloses
absorbent cores with a patterned layer of SAP immobilized by a net
of fibrous thermoplastic adhesive material deposited over the layer
of SAP. The fibrous thermoplastic material helps maintaining the
SAP in position within the absorbent core prior to and during use
of the article, without substantially restricting the ability of
the SAP to absorb large volumes of urine.
The absorbent material is typically enclosed in a core wrap so that
the absorbent core maintains its integrity on the converting line
when it is assembled with the other components of the absorbent
article and during use of the absorbent article. The core wrap may
be formed by any material suitable for receiving and containing the
absorbent material. Typical core wrap materials are papers,
tissues, films, wovens, nonwovens, and laminate of any of these.
The core wrap may in particular be formed by a nonwoven web, such
as a carded nonwoven, spunbond nonwoven ("S") or meltblown nonwoven
("M"), and multilayer laminates of these. Spunmelt nonwovens having
a multilayer SM, SMS, or SMMS, or SSMMS structure are commonly
used. Nonwoven materials provided from synthetic fibers, such as
PE, PET and in particular PP, are typically used.
Many core wraps comprise two separate substrates that are attached
to another at the edges of the core. The substrate oriented towards
the topsheet is referred herein as top layer and the substrate
oriented towards the backsheet is herein referred to as bottom
layer. It is also possible as an alternative to have a core wrap
comprising a single substrate that forms at the same time the top
layer and the bottom layer. The present disclosure is applicable to
both type of core wrap construction.
The top layer of the core wrap is typically hydrophilic, so that
the urine can quickly pass through this top layer and be absorbed
by the absorbent material. Nonwoven layers made of synthetic fibers
are typically rendered hydrophilic by adding a surfactant at their
surface, as is known in the art. The bottom layer may be
advantageously more hydrophobic than the top layer, to provide
additional barrier properties in addition to the backsheet.
Synthetic fibers are typically inherently hydrophobic unless
treated as indicated above.
Typical attachments between the core wrap layers are the so-called
C-wrap and the sandwich seal. In a C-wrap, the longitudinal and/or
transversal edges of one of the layer form flaps that extend beyond
the edges of the absorbent material area and are folded over the
other layer. These folded flaps are bonded to the external surface
of the other layer, typically by gluing. In the sandwich seal,
extensions of both layers are attached to each other in a face to
face relation at the edge of the core. Both types of attachments
may be combined, for example C-wraps on the longitudinal sides of
the core and a sandwich seal at the transversal edges. If the core
wrap is made of a single substrate, a C-wrap is typically used
longitudinally for a single longitudinal seal and optionally a
sandwich seal at each of the transversal edges.
It is generally desirable to reduce the manufacturing costs of
disposable diapers. One way to reduce cost is to reduce the basis
weight of the materials used. It has been however proven difficult
to reduce the basis weight of nonwoven core wrap material below
about 10 gsm due to irregularities known as wrinkling happening on
the converting line, especially taking into account the high speed
encountered on modern converting machines. These wrinkles can
develop in so-called z-folds that create mistracked web rejects and
line stops, especially just before splicing. The present disclosure
addresses this problem.
SUMMARY
The present disclosure in a first aspect for an absorbent article
is indicated in claim 1. The article comprises a liquid permeable
topsheet, a liquid impermeable backsheet, and an absorbent core
between the topsheet and the backsheet, wherein the absorbent core
comprises an absorbent material enclosed in a core wrap having a
top layer and a bottom layer. The bottom layer of the core wrap
comprises a nonwoven having a low basis weight of from about 6
g/m.sup.2 to about 10 g/m.sup.2 and has a static coefficient of
friction as measured in cross-machine direction of no more than
0.40. The coefficient of friction is measured according to standard
method ASTM D1894-01.
In a second aspect, the present disclosure is directed, in part, to
the use of a slip agent having Coefficient of Friction-reducing
properties to reduce the occurrence of wrinkles of the nonwoven
when the nonwoven is driven between two rollers. In a particular
aspect, the slip agent is added as a melt additive to a plastic
melt from which the nonwoven fibers are made, and then converting
the resulting nonwoven in a continuous process on a converting line
into an absorbent core. The nonwoven is used to form the bottom
layer of the absorbent core, the absorbent core further comprising
an absorbent layer and a top layer, the top layer and the bottom
layer forming a core wrap around the absorbent material. The melt
additive can be a fatty acid amide, as a single compound or as a
blend of different fatty acid amides.
The present disclosure is directed, in part, to an absorbent
article having a wearer-facing surface. The absorbent article
comprises a liquid permeable topsheet, a liquid impermeable
backsheet, and an absorbent core at least partially between the
topsheet and the backsheet. The absorbent core comprises an
absorbent material enclosed in a core wrap comprising a first layer
and a second layer. The first layer is positioned more proximate to
the wearer-facing surface than the second layer. The second layer
of the core wrap comprises a nonwoven comprising synthetic fibers.
The nonwoven has a basis weight from about 6 g/m.sup.2 to about 10
g/m.sup.2. The nonwoven has a static coefficient of friction as
measured in cross-machine direction of no more than 0.40, as
measured according to ASTM D1894-01. At least some of the fibers of
the nonwoven comprise a slip agent that reduces the coefficient of
friction of the nonwoven.
BRIEF DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims particularly pointing
out and distinctly claiming the subject matter which is regarded as
forming the present invention, it is believed that the invention
will be better understood from the following description which is
taken in conjunction with the accompanying drawings in which the
designations are used to designate substantially identical elements
and in which:
FIG. 1 shows a perspective view of an exemplary taped diaper in a
closed configuration as it would be when worn by a wearer;
FIG. 2 shows the garment-facing side of the diaper of FIG. 1 with
the diaper flattened out;
FIG. 3 shows the wearer-facing side of the diaper of FIG. 1 with
the diaper flattened out;
FIG. 4 shows a top view of an exemplary absorbent core with the top
layer partially removed;
FIG. 5 shows a longitudinal cross-section view of the absorbent
core of FIG. 4;
FIG. 6 shows transversal cross-section view of the absorbent core
of FIG. 4;
FIG. 7a, b, c, d illustrate the different stages of formation of a
fold-over 730 for a nonwoven being driven on a cylindrical roller;
and
FIG. 8 is a schematic representation of the test stand used in the
experimental section.
DETAILED DESCRIPTION
Various non-limiting forms of the present disclosure will now be
described to provide an overall understanding of the principles of
the structure, function, manufacture, and use of the absorbent core
wrap with a low basis weight nonwoven in the bottom layer disclosed
herein. One or more examples of these non-limiting forms are
illustrated in the accompanying drawings. Those of ordinary skill
in the art will understand that the absorbent core wrap with a low
basis weight nonwoven in the bottom layer specifically described
herein and illustrated in the accompanying drawings are
non-limiting example forms and that the scope of the various
non-limiting forms of the present disclosure are defined solely by
the claims. The features illustrated or described in connection
with one non-limiting form may be combined with the features of
other non-limiting forms. Such modifications and variations are
intended to be included within the scope of the present
disclosure.
General Description of an Absorbent Article
An exemplary absorbent article according to the present disclosure
in the form of a baby taped diaper 20 is represented in FIGS. 1-3.
FIG. 1 is a perspective view of the exemplary diaper in a closed
state as it would appear when worn by a wearer. This taped diaper
20 is shown for illustration purpose only as the present disclosure
may be used for making a wide variety of diapers or other absorbent
articles such as baby diaper pants, adult incontinence pants or
feminine sanitary pads. In the following, the word "diaper" and
"absorbent article" are used interchangeably. The Figures are used
herein as illustration of one way to carry out the present
disclosure and are not limiting the scope of the claims, unless
specifically indicated to do so.
The absorbent article comprises a liquid permeable topsheet 24 on
its wearer-facing surface, a liquid impermeable backsheet 25 on its
garment-facing surface and an absorbent core 28 between the
topsheet and the backsheet (shown in dotted line in FIG. 2). The
topsheet typically forms the majority of the wearer-contacting
surface of the article and is the first layer that the body
exudates contact. The topsheet is liquid permeable, permitting
liquids to readily penetrate through its thickness. Any known
topsheet may be used in the present disclosure. The backsheet
typically comprises a fluid impermeable plastic film, which may be
printed with a backsheet pattern, and a low basis weight nonwoven
cover glued to this impermeable film to give a nicer feel and
appearance to the backsheet.
The absorbent article may also comprise a fluid acquisition layer
and/or a fluid distribution layer between the topsheet and the
absorbent core and other typical diaper components such as
elasticized gasketing cuffs, upstanding barrier leg cuffs, which
are not represented for simplicity but are present in most diapers.
The absorbent article may also comprise other usual components if
it is desired to increase the performance of the article, such as
transverse barrier cuffs, front and/or back elastic waistbands, a
lotion application on the topsheet, longitudinally extending
channels in the core and/or the distribution layer, a wetness
indicator, etc. . . . all these components have been described and
exemplified in the art and are not further detailed herein. More
detailed disclosures of example of such components are for example
disclosed in WO201493323, WO2015/183669 (both Bianchi et al), WO
2015/031225 (Roe et al.) or WO2016/133712 (Ehrnsperger et al.) to
name a few.
The absorbent article typically comprises a front edge 10, a back
edge 12, and two longitudinally-extending side (lateral) edges 13,
14. The front edge 10 is the edge of the article which is intended
to be placed towards the front of the user when worn, and the back
edge 12 is the opposite edge. The topsheet 24, the backsheet 25,
the absorbent core 28 and the other article components may be
assembled in a variety of well-known configurations, in particular
by gluing, fusion and/or pressure bonding. The absorbent articles
of the present disclosure may comprises any typical layers and
components used in absorbent products of the diaper type, and which
are not necessarily represented in the simplified FIGS. 1-3.
General Description of an Absorbent Core
The absorbent core is the component of the absorbent article having
the most absorbent capacity. An exemplary absorbent core 28 is
shown in isolation in FIGS. 4-6, in dry state (before use). The
absorbent core represented has a generally rectangular shape as
defined by the longitudinal edges 284, 286 and transversal front
edge 280 and back edge 282. The periphery of the layer formed by
the absorbent material 60 within the core wrap, as seen from the
top side of the absorbent core, can be generally rectangular, for
example as shown in FIG. 4. This absorbent core represented is of
course not limiting the scope of the present disclosure as the
present disclosure is applicable to a wide variety of absorbent
cores. Other shapes can also be used such as a "T" or "Y" or
"sand-hour" or "dog-bone" shape for the area of the absorbent
material. In particular the area of absorbent material may define a
tapering along its width towards the middle region of the core. In
this way, the absorbent material deposition area may have a
relatively narrow width in an area of the core intended to be
placed in the crotch region of the absorbent article. This may
provide for example better wearing comfort.
The absorbent material 60 may be any conventional absorbent
material known in the art. For example the absorbent material may
comprise a blend of cellulose fibers and superabsorbent particles
("SAP"), typically with the percentage of SAP ranging from 40% to
70% by weight of the absorbent material. The absorbent material may
also be free of cellulose fibers, as is known in so-called
airfelt-free cores where the absorbent material consists of SAP.
"Superabsorbent polymers" or "SAP" as used herein refer to
absorbent material which are cross-linked polymeric materials that
can absorb at least 10 times, optionally at least 15 times, their
weight of an aqueous 0.9% saline solution as measured using the
Centrifuge Retention Capacity (CRC) test (EDANA method WSP
241.2-05E). These polymers are typically used in particulate forms
so as to be flowable in the dry state. The term "particles" refers
to granules, fibers, flakes, spheres, powders, platelets and other
shapes and forms known to persons skilled in the art of
superabsorbent polymer particles.
Various absorbent core designs comprising high amount of SAP have
been proposed in the past, see for example in U.S. Pat. No.
5,599,335 (Goldman), EP1,447,066 (Busam), WO95/11652 (Tanzer),
US2008/0312622A1 (Hundorf), WO2012/052172 (Van Malderen). In
particular the SAP printing technology as disclosed in
US2006/024433 (Blessing), US2008/0312617 and US2010/0051166A1 (both
to Hundorf et al.) may be used. The present disclosure is however
not limited to a particular type of absorbent core. The absorbent
core may also comprise one or more glue such as auxiliary glue
applied between the internal surface of one (or both) of the core
wrap layers and the absorbent material to reduce leakage of SAP
outside the core wrap. A micro-fibrous adhesive net may also be
used in air-felt free cores as described in the above Hundorf
references. These glues are not represented in the Figures for
simplicity.
The absorbent material may be for example deposited as a continuous
layer within the core wrap. The absorbent material may also be
present discontinuously for example as individual pockets or
stripes of absorbent material enclosed within the core wrap and
separated from each other by material-free junction areas. A
continuous layer of absorbent material, in particular of SAP, may
also be obtained by combining two absorbent layers having matching
discontinuous absorbent material application pattern wherein the
resulting layer is substantially continuously distributed across
the absorbent particulate polymer material area. As for example
taught in US2008/0312622A1 (Hundorf), each absorbent material layer
may thus comprise a pattern having absorbent material land areas
and absorbent material-free junction areas, wherein the absorbent
material land areas of the first layer correspond substantially to
the absorbent material-free junction areas of the second layer and
vice versa.
The basis weight (amount deposited per unit of surface) of the
absorbent material may also be varied to create a profiled
distribution of absorbent material, in particular in the
longitudinal direction (as schematically illustrated in FIG. 5) to
provide more absorbency towards the center and the middle of the
core, but also in the transversal direction, or both directions of
the core. The absorbent core may also comprise longitudinally
extending channels which are substantially free of absorbent
material within the absorbent material area. The core wrap may be
bonded through these material-free areas. Exemplary disclosures of
such channels in an airfelt-free core can be found in WO2012/170778
(Rosati et al.) and US2012/0312491 (Jackels). Channels may of
course also be formed in absorbent cores comprising cellulose
fibers.
Core Wrap
The function of the core wrap is to enclose the absorbent material.
As indicated in the background, different core wrap constructions
can be used. Typical core wraps comprise two substrates 16, 16'
which are attached to another and form respectively the top layer
and the bottom layer of the core wrap. These two layers may be
typically attached to another along at least part of the periphery
of the absorbent core to form a seal. Typically neither the first
nor the second substrate needs to be shaped, so that they can be
rectangularly cut for ease of production but other shapes are not
excluded. The terms "seal" and "enclosing" are to be understood in
a broad sense. The seal does not need to be continuous along the
whole periphery of the core wrap but may be discontinuous along
part or the whole of it, such as formed by a series of seal points
spaced on a line. Typically a seal may be formed by gluing and/or
thermal bonding.
The core wrap represented in the Figures comprises a top layer 16
which is wider than the bottom layer 16' so that two flaps of the
top layer can be folded over the bottom layer along the
longitudinal edges 284, 286 of the core respectively to which they
are attached, typically by an adhesive to form the longitudinal
seals 284', 286'. The front edge 280 and back edge 282 may also be
sealed, for example by a sandwich seal 280', 282'. Such transversal
seals may for example made by adhesive stripes applied by the slot
glue technique, as is known in the art. Alternatively, is it
possible to leave the transversal edges 280, 282 open without a
seal. For example there may be enough core wrap material between
the edges of the core and the absorbent material 60 to provide a
buffer zone at these ends.
The top layer and the bottom layer may be made from the same base
substrate material which has been differently treated. As indicated
in the background, the top layer may be typically a nonwoven layer
made of synthetic fibers that has been treated with a surfactant to
increase its hydrophilicity. Hydrophilicity and wettability are
typically defined in terms of contact angle and the strike through
time of the fluids. This is discussed in detail in the American
Chemical Society publication entitled "Contact angle, wettability
and adhesion", edited by Robert F. Gould (Copyright 1964). A
substrate having a lower contact angle between water and its
surface is more hydrophilic than another having a higher contact
angle. Generally, if the water contact angle is smaller than
90.degree., the substrate is considered hydrophilic and if the
water contact angle is larger than 90.degree., the substrate is
considered hydrophobic. The contact angle values can be measured on
the fibers at the surface of the nonwoven using a SEM microscope as
indicated in the experimental section below.
The bottom layer and top layer may be made of a similar type of
nonwoven, but with a different treatment to provide the different
desired properties. Both layers may in particular each comprises or
consists of a nonwoven web, such as a carded nonwoven, a spunbond
nonwoven ("S") or a meltblown nonwoven ("M"), and a multi-layer of
any of these. For example spunbond/meltblown laminate (spunmelt)
polypropylene nonwovens are commonly used and are particularly
suitable, especially those having a multi-layer SMS, or SMMS, or
SSMMS, structure. Examples are disclosed in U.S. Pat. No.
7,744,576, US2011/0268932A1, US2011/0319848A1 or US2011/0250413A1.
Typical material used to make the synthetic fibers are PE
(polyethylene), PET (polyethylene terephthalate) and in particular
PP (polypropylene). As will be discussed in details below, the
fibers of the nonwoven for the bottom layer can comprise a melt
additive that reduces the coefficient of friction of the nonwoven,
in particular a fatty acid amid additive. The bottom layer is also
typically not treated by a surfactant unlike the top layer of the
core wrap.
Nonwoven fabrics (herein referred to as "nonwovens") are sheet or
web structures bonded together by entangling fiber or filaments
mechanically, thermally, or chemically. They are flat, porous
sheets that are made directly from separate fibers. They are not
made by weaving or knitting and do not require converting the
fibers to yarn. Nonwoven webs can be formed by many processes such
as meltblowing, spunbonding, solvent spinning, electrospinning,
carding and airlaying. The basis weight of nonwoven webs is usually
expressed in grams per square meter (g/m.sup.2 or gsm).
Spunbond, also called spunlaid, nonwovens are made in one
continuous process. Fibers are spun through a number of small
orifices in a spinneret to form fibers or filaments, which are then
directly dispersed into a web by deflectors or can be directed with
air streams on a moving foraminous surface, such as a wire mesh
conveyor. Meltblown nonwovens are produced by extruding melted
polymer fibers through a spinneret or die consisting of up to 40
holes per inch to form long thin fibers which are stretched and
cooled by passing hot air over the fibers as they fall from the
die. The diameters of the fiber is significantly reduced by hot air
which also breaks the continuous filaments into microfibers of
varying length to diameter ratio. The extremely fine fibers
(typically polypropylene) differ from other extrusions,
particularly spunbond, in that they have low intrinsic strength but
much smaller size offering key properties.
The spunbond process can be combined with the meltblown process to
form a multi-layer web having S (spunbond) layer and M (meltblown)
layer, in particular SM, SMS or SMMS webs, which are strong and
offer the intrinsic benefits of fine fibers. The nonwovens may be
consolidated using known techniques, typically thermal point
bonding. In thermal point bonding, heat is applied locally on
individual regions of the nonwoven to locally melt and fuse the
fibers together. Fusion bond patterns are for example disclosed in
US 2011/0250413 (Hu et al.) and US2014/0072767A1 (Klaska et al.).
The resultant web is typically collected into rolls at the supplier
and subsequently converted to finished products.
Wrinkling
As indicated in the background, it is generally desirable to reduce
the basis weight of the core wrap layers. While experimenting with
core wrap layers both made with SMS nonwovens having a reduced
basis weight of about 8 gsm, the present inventors have
surprisingly found that a process anomaly known as wrinkling was
more likely to happen at the top layer than at the bottom layer of
the core wrap. Winkling can occur at the surface of a nonwoven when
it is driven between two rollers on a converting line, and can lead
to a fold-over that necessitate to stop the line. Both layers had
the same composition and basis weight and were submitted to the
same speed and tension. The difference between the top layer and
the bottom layer was that the top layer had been treated by a
surfactant to make it more hydrophilic, as is known in art.
While not wishing to be bound by theory, the present inventors
believe that the wrinkle formation is caused by thin and thick
spots in a web, due to difference in fiber orientation and basis
weight variation. This is particularly a problem for low basis
weight nonwovens such as those claimed. These irregularities can
induce shear stress in the nonwoven in the converting line
resulting in localized cross-machine direction (CD) buckling. These
wrinkles can further develop in z-folds that force the line to stop
or more desirably may resorb back to normal. This evolution is
schematically illustrated in FIG. 7a-d.
FIG. 7a shows a cross-section of a cylindrical roller 700 on top of
which a continuous web of nonwoven 16' is driven at a given speed
and tension. The nonwoven web is flat on the roller and is driven
under optimal conditions.
FIG. 7b illustrates the appearance of troughes, which are waves in
the free span region between rollers of a cross-section of the web
perpendicular to the direction of web flow, with flat or planar web
where the web wraps around a roller. Troughing nearly always occurs
on commercial converting line and is as such no concern.
FIG. 7c illustrates the appearance of wrinkles that are developing
from the troughes. Wrinkles are separations of the non-woven from
the roller surface in a region where the web wraps around an idler.
Wrinkles are typical of planar buckling due to compressive stresses
which are substantially aligned with the cross-machine direction of
a web.
FIG. 7d illustrates next the development of a foldover, which is
three or more layers of web in any one or more locations in the
cross-machine direction, in the region where the web is wrapped
around a roller. Foldover requires to stop the line to correct the
issue.
Based on these findings, the inventors have hypothesized that
bringing the Coefficient of Friction (CoF) of the bottom layer to
the level of the top layer would lead to the same reduction of the
wrinkles as in the top layer. While not wishing to be bound by
theory, it is believed that the hydrophilic surfactant treatment
had as a secondary effect a reduction of the CoF of the top layer.
When the top layer is converted on the line, any wrinkles
developing from troughes were found to flatten out quickly after
their appearance. The CoF is traditionally used to measure the
facility at which one layer can slide over another layer of the
same material, and is measured according to the standard method
ASTM D1894-01 with the further indications as indicated below. The
lower the CoF is, the more the layers can slip over another. All
values indicated herein are for the static CoF measured in the
cross-machine (CD) direction of the nonwoven.
As will be shown below in the experimental section, the present
inventors have indeed found that by adding a slip agent to the
bottom layer, wrinkles can be efficiently prevented. The slip agent
used may be in particular a melt additive comprising fatty acid
amides. Fatty acid amides are known to migrate to the surface of
the fibers of the nonwoven (effect known as blooming) and thereby
reduce the coefficient of friction. As a further advantage, it is
believed that the hydrophobic properties of the nonwoven are
enhanced by the presence of the fatty acid amide. While the data
presented were generated with a fatty acid amide melt additive,
reducing the CoF of the nonwoven by other known means is also
applicable, some of which will be discussed below.
CoF Reducing Agents--Fatty Acid Amides
The nonwovens of the present disclosure have a static coefficient
of friction (CoF) as measured in cross-machine direction of no more
than 0.40 (measured according to ASTM D1894-01), in particular from
0.20 to 0.39, more particularly from 0.25 to 0.38, more
particularly from 0.30 to 0.38. This CoF can be in particular
obtained by adding to the fibers of the nonwoven a melt additive
that reduces the coefficient of friction of the nonwoven, or by any
other known slip agent for reducing the CoF of a nonwoven, such as
surface treatment with a slip agent. In addition to the bottom
layer, the top layer of the core wrap can also advantageously have
a CoF of no more than 0.40, in particular from 0.20 to 0.39, more
particularly from 0.25 to 0.38, more particularly from 0.30 to
0.38.
Oleamide and erucamide are fatty acid amides commonly used as slip
agent in polyethylene or polypropylene nonwovens. These compounds
can be introduced as melt additives in the molten plastic before
the making of the nonwovens. Oleamide is derived from
mono-unsaturated C18 oleic acid, while erucamide is the amide of
C22 mono-unsaturated erucic acid. The molecules of fatty acid
amides are known to migrate at the surface of the fibers and thus
provide a self-replenishing surface lubrication (effect referred to
as "blooming"). This has been proven to reduce the coefficient of
friction of the nonwovens made from these fibers or filaments. An
early disclosure of this effect is disclosed in U.S. Pat. No.
3,454,519 (Hulse et al.), which discloses improved textile fibers
prepared from isotactic polypropylene resin containing from about
0.01 to about 1.0% by weight of erucamide. Oleamide has a lower
basis weight and is known to migrate to the film surface more
rapidly than erucamide. Erucamide can however produce lower CoF
values than equal quantities of Oleamide. Stearamide is also known
to reduce the coefficient of friction when used as melt additive to
the polymer melt used to make the nonwoven's fibers. Blend of such
fatty acids may also be used. Such a blend of Stearamide and
erucamide is for example disclosed in U.S. Pat. No. 6,740,609 (Peng
et al.).
Typically the heat generated during the making of the nonwoven and
on the diaper converting line may be enough for the blooming to
occur. The migration of the molecules of fatty acid amides may also
be accelerated up by heating the nonwoven at 40-50.degree. C. for a
few hours if necessary. The heat can be inserted via the
thermo-bonding step as is commonly used in the production of
nonwoven. Typically, the web is wound into rolls within a few
seconds after the thermo-bonding step. Typically under these
conditions a temperature of around 40.degree. C. is maintained over
several hours inside the rolls. It has been found that the
insertion of an additional heating step, e.g. by using standard
drying equipment (such as for drying surfactant coatings), does not
only accelerate the blooming of erucamide but also enables an
overall value of the coefficient of friction. This can be used for
further usage reduction of erucamide. However, it requires an
additional process operation which may not be available at the
production line and its costs further energy. So it needs to be
decided in each individual case if to insert an additional heating
step for further erucamide usage level reduction.
Any fatty acid amide melt additives that can reduce the coefficient
of friction of the nonwoven may be used. Suitable fatty acid amides
include those derives from a mixture of C12-C28 fatty acids
(saturated or unsaturated) and primary or secondary amines. A
suitable example of a primary fatty acid amide includes those
derived from a fatty acid and ammonia as illustrated in [1].
##STR00001##
where R has a number of carbon atoms ranging from 11 to 27, in
particular from 16 to 22 fatty acid. Fatty acid amides have the
further advantage that they can increase the hydrophobicity and
with this its barrier properties of the nonwoven, which is
desirable for the bottom layer of the core wrap.
More generally, any treatment or melt additive that can reduce the
CoF of the nonwoven may be used. Other suitable hydrophobic melt
additives include hydrophobic silicones and ethoxylated fatty
alcohols. Additional suitable hydrophobic melt additives are
disclosed in US2016/067118 (Hammons et al.). A surfactant treatment
as in for the top layer may also be envisioned to reduce the CoF of
the bottom layer, however it may be desired that the bottom layer
keep its barrier properties. The treatment or additive used to
reduce to CoF may advantageously increase or at least maintain the
barrier properties of the bottom layer of the core wrap. The
hydrophobicity can be compared using the contact angle method, as
indicated above. The top layer of the core wrap can be
advantageously more hydrophilic than the bottom layer of the core
wrap.
As will be illustrated below, the present disclosure allows to use
low basis weight nonwovens as bottom layer for the core wrap,
wherein the occurrence of wrinkles is diminished or avoided. This
in turn ensures that foldover becomes even rare or is completely
avoided under normal conditions.
It has also been found that a lower COF helps in the reduction of
abrasion of small fibers (often referred to as "fuzz") from the
nonwoven. Fuzz can impede the nonwoven's integrity and cause
in-process contamination problems, besides an unpleasant visual
appearance and haptic perception of the nonwoven. The unpleasant
visual appearance of a nonwoven with significant fuzz may shine
through the backsheet or the unpleasant haptic perception may be
felt through the backsheet. Fuzz (loose small fibers) is generated
due to abrasion in the converting process (in contact of the web
with equipment parts, e.g. rollers, idlers, . . . ). This is a
challenge particularly with low basis weight nonwovens which have
an inherently lower strength and suffer more from in-process
abrasion. It is believed that with a lower CoF, mechanically
entangled filaments or fibers in the nonwoven are able to slide
against each other when a force is applied on them rather than
being plastically deformed until they tear. This enables in general
more abrasion resistant NWs. The blooming of melt additives is
known to be a thermodynamically driven process due to the
incompatibility of the melt additive in the polymer matrix and
happens very uniformly across all fibers in the web, as opposed to
an in nature more inhomogeneous topical coating step applying a
slip agent or lubricant onto the surface of the web. So the uniform
blooming of the melt additive within all nonwoven layers to which
it has been applied can enable a more effective reduction of fuzz
creation.
Test Methods
Coefficient of Friction (CoF)
The Static Coefficient of Friction (CoF) is measured using ASTM
Method D 1894-01 with the following particulars. The test is
performed on a constant rate of extension tensile tester with
computer interface (a suitable instrument is the MTS Insight using
Testworks 4 Software, as available from MTS Systems Corp., Eden
Prairie, Minn. or equivalent) fitted with a coefficient of friction
fixture and sled as described in D 1894-01 (a suitable fixture is
the Coefficient of Friction Fixture and Sled available from Instron
Corp., Canton, Mass., or equivalent). The apparatus is configured
as depicted in FIG. 1. c of ASTM 1894-01 using the same nonwoven
substrate as the target surface and the sled surface. A load cell
is selected such that the measured forces are within 10-90% of the
range of the cell. The tensile tester is programmed for a crosshead
speed of 127 mm/min, and a total travel of 130 mm. Data is
collected at a rate of 100 Hz. All testing is performed in a room
where the temperature is controlled at about 23.degree.
C..+-.2.degree. C. and 50%.+-.2% relative humidity with all samples
and nonwoven targets conditioned under the same conditions for at
least two (2) hours prior to testing.
The nonwoven is harvested from the absorbent article using
cryogenic freeze spray (e.g., Cyto-Freeze available from VWR) or
other appropriate means to separate the layers. Ensure that the
orientation of the test specimen is maintained, i.e., cross
direction CD versus machine direction MD, and embossed surface
versus non-embossed surface if applicable. Cut a test specimen 8.9
cm by 8.9 cm from the sample substrate with its cut sides parallel
and perpendicular to the machine direction of the substrate. Mount
the specimen onto the foam rubber side of the sled by wrapping the
edges around to the back of the sled and securing with adhesive
tape. The specimen is oriented such that the specimen will be
pulled along the cross direction CD of the specimen during the test
and that the embossed surface is directed toward the target
surface.
For the nonwoven target, cut a specimen approximately 8.0 cm by
20.0 cm with the substrate oriented such that the cross direction
CD is parallel with the direction of the sled pull and the embossed
surface is facing the sled's surface. The nonwoven target is
secured to the platform using masking tape around its circumference
such that the no tape interferes with the movement of the sled
during testing. A new nonwoven target and sled surface is used for
each measurement.
Set up the tensile test as described above. Zero the load cell and
crosshead. Connect the sled to the lead line and place the sled,
specimen surface down, onto the target plane. The line should be
secure under the pulley and taut, with less than 1.0 g force on the
load cell. Start the test and collect force verses distance data.
When the test is complete, remove the specimen from the sled and
target. From the resulting extension (mm) versus force (gf) graph
calculate the Static and Kinetic Coefficient of Friction as
follows: Static CoF=maximum peak force (gf) for the initial peak
divided by the measured mass (g) of the sled and recorded to the
nearest 0.01.
The test is repeated for a total of ten (10) replicates. Average
values are calculated separately for Static CoF for each of the
specified targets and reported to the nearest 0.01 units.
SEM Method for Determining Contact Angle on Fibers
A rectangular specimen measuring 1 cm.times.2 cm is cut from the
middle of the material to be tested with the length of the specimen
(2 cm) aligned with a longitudinal centerline of the article. The
specimen is handled gently by the edges using forceps and is
mounted flat with the skin-facing side up on an SEM specimen holder
using double-sided tape. The specimen is sprayed with a fine mist
of water droplets generated using a small hobby air-brush
apparatus. The water used to generate the droplets is distilled
deionized water with a resistivity of at least 18 Me-cm. The
airbrush is adjusted so that the droplets each have a volume of
about 2 pL. Approximately 0.5 mg of water droplets are evenly and
gently deposited onto the specimen. Immediately after applying the
water droplets, the mounted specimen is frozen by plunging it into
liquid nitrogen. After freezing, the sample is transferred to a
Cryo-SEM prep chamber at -150.degree. C., coated with Au/Pd, and
transferred into Cryo-SEM chamber at -150.degree. C. A Hitachi
S-4700 Cry-SEM or equivalent instrument is used to obtain
high-resolution images of the droplets on the fibers. Droplets are
randomly selected, though a droplet is suitable to be imaged only
if it is oriented in the microscope such that the projection of the
droplet extending from the fiber surface is approximately
maximized. The contact angle between the droplet and the fiber is
determined directly from the images taken as is known in the art.
Twenty separate droplets are imaged from which forty contact angle
measurements are performed (one on each side of each imaged
droplet), and the arithmetic average of these forty contact angle
measurements is calculated and reported as the contact angle for
that specimen.
Experimental
Test Nonwovens
Different nonwovens (NW) having the same SMMS laminate structure
and same 8 gsm basis weight were made using a conventional process
as is known in the art. The spun to melt weight ratio was around
80/20. Thermal bonds were applied via a heated calendering roll to
the nonwovens as is known in the art. A commercially available oval
bond pattern (Ungrich U2888) was used. The reference nonwoven was
not further treated. One nonwoven was hydrophillically treated by a
surfactant as would be done for a top layer of the core wrap. The
other nonwovens had different amount of erucamide introduced as hot
melt additive in the S layers at 0.1%, 0.3% and 0.5% by weight of
the SMMS nonwoven. The Erucamide was added by extrusion through a
10% masterbatch containing 10% of active Erucamide. The Erucamide
was added into both S-beams. The weight percentage of the hotmelt
additive refers to the total mass of the NW.
The static CoF in CD direction were measured on 10 randomly
selected samples for each nonwoven. The mean and the standard
variation values are reported below.
TABLE-US-00001 Mean Std Deviation Reference (no surfactant, no
erucamide) 0.51 0.069 Reference with a surfactant treatment 0.385
0.019 Reference + 0.1% Erucamide 0.394 0.031 Reference + 0.3%
Erucamide 0.329 0.019 Reference + 0.5% Erucamide 0.341 0.033
It is known that some heat introduction is advantageous for the
blooming of erucamide out of PP fibers (typically a few hours at
40.degree. C.). However, no additional heat treatment in the test
nonwovens was found necessary for the erucamide to bloom out at the
surface of the fibers, presumably due to the stored heat in the
calendering roll. In production processes, enough heat can also be
provided via the thermo-bonding (calendering) step of the NW and
subsequent winding of the NW on rolls whereby residual process heat
is stored inside the rolls.
As can be seen above, already introducing 0.1% Erucamide was found
to be sufficient to lower the CoF of the bottom layer to about the
same level as a hydrophillically treated nonwoven that can be used
as top layer.
0.3% Erucamide was found to lower even more the CoF. Increasing
further the level of Erucamide to 0.5% by weight did not further
decrease the CoF significantly in this experiment.
Test Stand:
The 8 gsm SMMS untreated reference nonwoven (NW) and the inventive
8 gsm SMMS NW containing 0.3% erucamide were tested on a static
test stand that simulates a portion of a diaper converting line.
The test stand 800 is schematically represented on FIG. 8, with the
following reference numbers indicated the following elements:
810=roll of the nonwoven to be tested, 820=idlers that loop the web
back over the unwind roll, 830=dancer to control tension 840=Idler
roll on a linear stage (the distance between 840 and 850 is the
webspan and can be varied, the web tension is also between these
two idlers), 850=Test idler (a concave idler was used in this
study) observed by a camera (represented by the lighting) to
analyze the presence and formation of wrinkles and other anomaly,
860=wind-up roll.
The 3-idlers system 820 lessen any wound up tensions prior to going
into the dancer 830 that has the two idlers on the left able to
swing. The concave idler 850 used in the test design had a V-shape
concavity (with a diameter of 50 mm at the edges and a depth of the
valley of 800 .mu.m, i.e. a diameter of (50 mm-2*800 .mu.m) in the
center). Before the wind-up roll 860, the web path can also use a
dancer similar to 830 to control tension (not represented in FIG.
8) in the rewinding.
Different typical tensions and web spans, which play a role in
production processes, e.g. in a splicing unit in the production
line, were tested. The presence or absence of wrinkles was recorded
with a camera on top of the idler (this test stand did not mimic
the actual--dynamic--splicing process, during which tensions and
web spans are ramping up and down, but represents static corner
points of a splicing process). The absence of "wrinkles" is
indicative of a smooth conversion in production processes,
particularly during the splicing step, while the presence of
wrinkles is indicative of convertibility problems production
processes, particularly during the splicing step. Convertibility
problems occur when wrinkles grow into fold-overs which are
irreversible and ultimately cause line stops.
Definitions
Troughing: Waves in the free span region between rollers of a
cross-section of the web perpendicular to the direction of web
flow, with flat or planar web where the web wraps around a roller.
Troughing nearly always occurs and is as such no concern.
Wrinkling: Separation of the non-woven from the roller surface in a
region where the web wraps around an idler. Typical of planar
buckling due to compressive stresses which are substantially
aligned with the cross-machine direction of a web.
Foldover: Three or more layers of web in any one or more locations
in the cross-machine direction, in the region where the web is
wrapped around a roller.
Test Results
TABLE-US-00002 Set Set Webspan Tension [N] [mm] Web Material
Observation 7.4 (.+-.1.4) 720 Inventive NW (0.3% Erucamide)
Troughing 3.5 (.+-.1.1) 1100 Inventive NW (0.3% Erucamide)
Troughing 7.4 1100 Inventive NW (0.3% Erucamide) Troughing 3.5 720
Inventive NW (0.3% Erucamide) Troughing 7.4 720 Reference NW
Wrinkling 3.5 1100 Reference NW Wrinkling 7.4 1100 Reference NW
Wrinkling 3.5 720 Reference NW Wrinkling
Whenever "wrinkling" was reported, 1 to 2 discrete wrinkle
formation events where observed within a time span of 10 s, when
looking (with the camera) at the idler.
Packages
Absorbent articles for personal hygiene are typically packaged by
the manufacturer in a plastic bag and/or a cardboard box for
transport and sale. The articles may also be folded before being
packaged to save space as is known in the art. The back and front
ears of taped diapers are for example typically folded inwardly
before bi-folding the diaper along its transversal axis before
being packaged. The absorbent articles may be packaged under
compression, so as to reduce the size of the package so that the
caregivers can easily handle and store the packages, while also
providing distribution and inventory savings to manufacturers owing
to the size of the packages. The package may for example comprise
from 2 to 200 of the articles.
Unless indicated otherwise, the description and claims refer to the
absorbent core and article before use (i.e. dry, and not loaded
with a fluid) and conditioned at least 24 hours at 21.degree.
C.+/-2.degree. C. and 50+/-5% Relative Humidity (RH).
The dimensions and values disclosed herein are not to be understood
as being strictly limited to the exact numerical values recited.
Instead, unless otherwise specified, each such dimension is
intended to mean both the recited value and a functionally
equivalent range surrounding that value. For example, a dimension
disclosed as "40 mm" is intended to mean "about 40 mm".
Every document cited herein, including any cross referenced or
related patent or application and any patent application or patent
to which this application claims priority or benefit thereof, is
hereby incorporated herein by reference in its entirety unless
expressly excluded or otherwise limited. The citation of any
document is not an admission that it is prior art with respect to
any invention disclosed or claimed herein or that it alone, or in
any combination with any other reference or references, teaches,
suggests or discloses any such invention. Further, to the extent
that any meaning or definition of a term in this document conflicts
with any meaning or definition of the same term in a document
incorporated by reference, the meaning or definition assigned to
that term in this document shall govern.
While particular forms of the present invention have been
illustrated and described, it would be obvious to those skilled in
the art that various other changes and modifications can be made
without departing from the spirit and scope of the invention. It is
therefore intended to cover in the appended claims all such changes
and modifications that are within the scope of this invention.
* * * * *